1. Field of the Invention
This invention relates to bolted joints, for use in low thermal expansion materials, and more particularly to a joint assembly wherein a metallic fastener joins material having a low coefficient of thermal expansion to a material having a high coefficient of thermal expansion.
2. Description of the Related Art
With the increasing use of ceramic and other low thermal expansion materials, a problem is encountered when such materials are joined to high thermal expansion materials. Such materials also are typically used in high strength conventional fasteners, such as metallic bolts. Metal fasteners are thermally incompatible with ceramic and other low thermal expansion materials in that the metal expands more than the ceramic material does with an increase in temperature. The coefficient of thermal expansion for metals ranges from 3.times.10.sup.-6 to 13.times.10.sup.-6 in./in./.degree.F., with the coefficient of thermal expansion for steel being about 10.times.10.sup.-6 in./in./.degree.F. On the other hand, ceramic materials have a coefficient of thermal expansion of 1.times.10.sup.-6 to 2.times.10.sup.-6 in./in./.degree.F. If a metallic bolt shank is fitted closely within a bore in a ceramic material, cracking of the ceramic material is likely to occur if the joint is exposed to temperature changes.
Various bushings are conventionally used for providing a close fit between a shank and a bore through which the shank passes. Two such bushings are disclosed in U.S. Pat. Nos. 4,156,299 and 3,643,290. However, these devices do not address the problem of differential thermal expansions. Other attempts have been made for joining ceramic and metal parts in a manner that compensates for differential thermal expansions. However such joints have proved complex and costly and they do not eliminate thermally induced stress in the materials being joined.
The problem of fitting a high thermal expansion fastener in the bore of a low thermal expansion material and the problem of compensating for differential expansions between the high and low thermal expansion materials being joined so that thermal stress does not result upon temperature change has not been addressed. The present invention provides a cost efficient solution to this problem.